1. Field of the Invention
The present invention relates to a low-threshold semiconductor laser of a minute cavity (or resonator) type which is usable for optical interconnections, parallel data processing, large-capacity parallel optical transmission and so forth, and particularly to a surface emitting semiconductor laser of a ring cavity type wherein resonance is generated in its layering direction, a fabrication method thereof, etc.
2. Related Background Art
With surface emitting semiconductor lasers, a two-dimensional arraying is possible with high density, and a low-threshold driving is possible as well. Because of those features, the surface emitting semiconductor laser is well-suited and popular as a light source in technical fields, such as optical interconnections, parallel optical data processing and large-capacity parallel optical transmission, and its development has been advanced noticeably.
The surface emitting semiconductor laser is generally comprised of a Fabry-Perot cavity with a cavity length of several microns which is formed of a pair of reflection mirrors. The surface emitting laser is roughly classified into two types depending on the kind of its substrate: one is a GaAs-series surface emitting laser with an oscillation wavelength of 0.85 .mu.m or 0.98 .mu.m, and the other is an InP-series surface emitting laser with an oscillation wavelength of 1.3 .mu.m or 1.55 .mu.m. Further, in order to promote a low threshold, it is required that the reflection mirror is as transparent as possible to the oscillation wavelength and has as high a reflectivity as possible. For those purposes, a multi-layer mirror is ordinarily used, which is formed by alternately depositing two kinds of materials having different indices of refraction with a thickness of a quarter of the oscillation wavelength.
In the case of a GaAs series surface, an AlAs/(Al)GaAs multi-layer, which can be epitaxially grown on the GaAs substrate, is generally used as a mirror. On the other hand, in the case of an InP series surface, since the refractive-index difference between InGaAsP/InP, which can be epitaxially grown on the InP substrate, is small and a high reflectivity is hard to obtain thereby, other materials, such as a SiO.sub.2 /Si multi-layer and an Al.sub.2 O.sub.3 /Si multi-layer, are generally used. Furthermore, the following method is also known. An AlAs/(Al)GaAs multi-layer, which is grown on the GaAs substrate, is fused with semiconductor layers including an active layer, which are separately grown on the InP substrate.
In the case of the GaAs series surfaces of the prior art surface emitting semiconductor lasers, however, layering of more than twenty pairs of AlAs and (Al)GaAs layers is needed to obtain a high reflectivity when using the AlAs/(Al)GaAs multi-layer, and therefore, much time is required to grow that multi-layer.
On the other hand, in the case of InP series surfaces, the SiO.sub.2 /Si multi-layer or the Al.sub.2 O.sub.3 /Si multi-layer has to be grown by a method in which deposition and sputtering are performed to form this multi-layer after semicondoctor layers are processed using the etching and the like. Thus, its fabrication process is complicated. Further, the thermal conductivity of SiO.sub.2 is small, and the thermal conductivity of the Si film, which is formed by deposition and sputtering, is also small, compared to that of a single crystal of Si. Hence, the thermal characteristics or thermal radiation efficiency of a laser with those materials are not what could be desired.
Further, where the AlAs/(Al)GaAs multi-layer is fused with the semiconductor layers, its thermal conductivity is relatively large, compared to those of the SiO.sub.2 /Si multi-layer and the Al.sub.2 O.sub.3 /Si multi-layer. However, since the fusion method is used, the process is complicated. Moreover, since the interface formed by the fusion is in the cavity, the condition of the interface (such as its cleanness and the junction strength) are likely to directly influence the laser characteristics.
Further, in the multi-layer on the p-side, it is difficult to lower its resistance even if doping of the layer is performed, because a number of hetero-barriers exist therein. Therefore, the electrode must be formed in such manner that an injected current can shun the multi-layer.